There are some grains that contribute sugars to the wort that are "less" fermentable than others. Crystal malts add sweetness not just because of the flavor they impart but also the sugars they contribute tend to be less fermentable than pale malt or 2 row. I have not heard this is the same case for flaked malts.
Sugar, honey, maple syrup and a few others are "more" fermentable than typical malt.
Think of it this way, if x amount of malt gives an og of 1.050 and the yeast attenuates it down to an fg of 1.015, the same scenario with crystal malts replacing a small portion of the pale malt will cause the FG to drop only to 1.017. Replace the crystal with sugar and the FG will drop to 1.012. This is just an example and the numbers are pulled from thin air but hopefully that illustrates it.
Just because you have sugars in the wort, doesn't mean they are all created (and fermented) equally.